1. Field of the Invention
The invention relates to a fluid pressure actuator of the linear type for effecting the shifting of a valve stem from an open to a closed position.
2. History of the Prior Art
In the production of oil and/or gas from subterranean wells, a large number of fluid pressure actuated valves are employed as safety valves, head valves, pipeline valves, etc. Many of such actuators are designed for actuation by air or gas at 250 psi. In recent years, the pressure rating for the valves have increased to the order of 20,000 to 30,000 psi, thus necessitating a very elaborate and very accurately machined sealing assemblies around the stem of the valve. Such sealing assemblage generally involves the maintenance of a minimal clearance between the valve stem and the seal elements mounted in the bonnet of the valve. As a result, forces on the order of 2,000 to 4,000 pounds of thrust are required to effect the initial movement of the valve stem relative to the high-pressure seal pack, regardless of what pressure exists in the valve. If the valve is under pressure, you obviously need still more thrust to be applied by the actuator.
Since most linearly actuated valves of this type are shiftable to either an open or closed position through the application of a fluid pressure to an actuator and are returned to the opposite position through the compression of a spring, then it is obvious that the springs employed must be capable of exerting thrust forces in excess of 4,000 pounds in order to effectively open or close the valve upon the removal of the actuating force on the valve stem. In light of physical size considerations, such spring forces inherently dictate the employment of Belleville disc springs, because only this type of spring can be axially stacked to generate a high thrust with a very short travel of the valve stem actuator. Once movement of the valve stem through the seal pack is initiated, it is highly desirable that the resultant movement of the valve stem be subject to hydraulic dampening in order to prevent damage to the actuator by the high decelerating forces generated in stopping the movement of the valve stem actuator in either direction. Furthermore, it is highly desirable that the stack of Belleville springs be disposed in a corrosion-free environment, such as being enclosed in an oil-filled reservoir to prevent rusting and, hence, embrittlement or weakening of the individual components of the stack of Belleville valve discs.